Water-absorbent structure and process for making the same

ABSTRACT

A water-absorbent structure comprises an assembly of thermoplastic synthetic fibers, cellulose-based fibers, super-absorbent polymer and water-pervious sheets wrapping these components together. The assembly is a honeycomb construction having a plurality of through-holes and normally in a compressed state. Upon water permeation into the water-absorbent structure, the super-absorbent polymer is swollen and softened and the assembly is swollen so that the through-holes restore the initial state before the assembly has been compressed.

BACKGROUND OF THE INVENTION

The present invention relates to a water-absorbent structure for variousarticles requiring a water-absorbing property such as a disposablediaper, a sanitary napkin and disposable wipes and also to a process formaking the same.

U.S. Pat. No. 4,500,315 discloses a disposable superthin water-absorbentarticle. This article comprises a first layer, a second layer and anintermediate layer disposed between these first and second layers. Thefirst layer comprises, in turn, a synthetic fibrous web and a pluralityof super-absorbent polymer particles distributed in this web. The secondlayer is a wad formed by intertwined cellulose fibers or the likecontributing to a liquid diffusibility. The intermediate layer also isformed by intertwined cellulose fibers or the like partially contactingwith the polymer particles of the first layer. This article iscompressed in its thickness direction so as to have a thickness lessthan half of the thickness before compressed and restores a thicknesscorresponding to at least 75% of its initial thickness before compressedas the article absorbs liquid such as bodily discharges.

Japanese Patent Application Publication No. 1990-74254A discloses anabsorbent pad used in the absorbent article. This absorbent padcomprises a mixture of heat-weldable crimped fibers, fluff pulp andwater-absorbent polymer particles. The crimped fibers are heat-weldedtogether to form three-dimensional web formation presenting a mat-likeappearance. The absorbent pad is obtained by compressing the fluff pulpand polymer particles in a wetted state together with the crimped fibersand then drying the assembly. The absorbent pad is free from a state ofcompression as the fluff pulp and the polymer particles absorb watersufficiently to become soft and thereupon the absorbent pad restores itsinitial mat-like state. After restoration of the initial mat-like state,the crimped fibers become easily deformable and, in consequence, theabsorbent pad acquires a compressive elasticity.

In both the article disclosed in U.S. Pat. No. 4,500,315 and theabsorbent pad disclosed in Japanese Patent Application Publication, thepolymer particles are filled and held in interstices of the fibers. Ifit is desired to avoid falling off of the polymer particles from theabsorbent article or pad, the fiber interstices must be as narrow aspossible. As a result, the fiber interstices are choked up as thepolymer particles absorb water and swell and a breathability of theabsorbent article or pad may be remarkably deteriorated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a water-absorbentstructure comprising a thermoplastic synthetic fiber assembly containinga water-absorbent material improved so that a desired breathability ofthe structure is maintained even when super-absorbent polymer containedin the structure absorb water and swell.

According to a first aspect of the invention, there is provided awater-absorbent structure, according to a second aspect of theinvention, there is provided a process for making the structure.

The first aspect of the present invention relates to a water-absorbentstructure comprising a panel-shaped assembly of thermoplastic syntheticfibers having upper and lower surfaces extending in parallel to eachother, cellulose-based fibers and super-absorbent polymer adapted to beswollen as the polymer absorbs water both contained in the assembly ofthe thermoplastic synthetic fibers served as a water-absorbing materialsin which at least one of the upper and lower surfaces is wrapped withwater-pervious sheets.

According to the invention, the assembly is in the form of a honeycombconstruction adapted to be elastically compressed in a thicknessdirection and has a plurality of through-holes extending parallel to oneanother in a direction parallel to the upper and lower surfaces, each ofthe through-holes has a cross-sectional dimension larger than any one ofinterstices of the thermoplastic synthetic fibers in the assembly. Theassembly is normally kept in a state compressed in the thicknessdirection with the through-holes being flattened and adapted to beelastically swollen in the thickness direction so that the flattenedthrough-holes are restored to the initial cross-sectional shape thereofas the super-absorbent polymer absorbs water and is swollen.

The invention on the first aspect includes the following embodiments.

The super-absorbent polymer is provided in particulate or fibrous form.

The assembly comprises a plurality of honeycomb thin leaves placed uponone another in a transverse direction in which the through-holes extend,each of the honeycomb thin leaves have a width of 3 to 30 mm as measuredin the transverse direction.

The through-holes in each pair of the thin leaves adjacent to each otherare at least partially connected.

The thermoplastic synthetic fibers are of crimped-type.

The assembly, the cellulose-based fibers and the super-absorbent polymerare mixed at a ratio of 5-80 wt %:5 -60 wt %:10-80 wt %.

A cross-sectional shape of the through-hole is a substantially rectangleand one of diagonals of the rectangle is substantially in coincidencewith the thickness direction.

The assembly includes at least two through-holes aligned in thethickness direction.

The second aspect of the invention relates to a process for making awater-absorbent structure comprising a panel-shaped assembly ofthermoplastic synthetic fibers having upper and lower surfaces extendingin parallel to each other, cellulose-based fibers and super-absorbentpolymer adapted to be swollen after absorption of water both containedin the assembly of the thermoplastic synthetic fibers served aswater-absorbing materials in which at least one of the upper and lowersurfaces is wrapped with water-pervious sheets.

According to the invention, the assembly is in the form of a honeycombconstruction adapted to be elastically ompressed in a thicknessdirection and has a plurality of through-holes extending parallel to oneanother in a direction parallel to the upper and lower surfaces, each ofthe through-holes has a cross-sectional dimension larger than any one ofinterstices of the thermoplastic synthetic fibers in the assembly. Theprocess comprises steps of compressing the assembly in the thicknessdirection while the water absorbing materials are in a wetted conditionso that the through-holes are flattened, drying the water absorbingmaterials so as to maintain the assembly in a compressed state andwrapping at least one of the upper and lower surfaces with thewater-pervious sheets before or after the step of compressing.

The invention on its second aspect includes the following embodiments.

The super-absorbent polymer is provided in particulate or fibrous form.

The process further comprises steps of feeding a mixture of thethermoplastic synthetic fibers, the cellulose-based fibers and thesuper-absorbent polymer into a molding die and welding the thermoplasticsynthetic fibers at crossways thereof within the molding die underheating to obtain the assembly.

The assembly includes at least two through-holes aligned in thethickness direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a water-absorbent structure;

FIG. 2A is a perspective view showing a stock panel and stock sheets;and FIG. 2B is a scale-enlarged diagram illustrating a part of FIG. 2A;

FIG. 3 is a perspective view showing the water-absorbent structure asafter it has absorbed water;

FIG. 4 is a view similar to FIG. 2 showing another embodiment of thestock panel; and

FIG. 5 is a partially cutaway perspective view showing a molding dieused to mold the stock panel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Details of a water-absorbent structure and a process for making the sameaccording to the present invention will be more fully understood fromthe description given hereunder with reference to the accompanyingdrawings.

A water-absorbent structure 1 shown in FIG. 1 in a perspective viewcomprises a panel-like core 2 and cover sheets 3 for the core 2. Thecover sheets 3 are indicated by imaginary lines. The core 2 comprises amixture of thermoplastic synthetic fibers 4, super-absorbent polymerparticles 6 and fluff pulp 7 compressed together and has an uppersurface 8, a lower surface 9 and a peripheral side surface 11. Thesheets 3 cover the core 2 on its upper, lower and side surfaces 8, 9, 11to prevent the core 2 from getting out of its shape and simultaneouslyto prevent the polymer particles 6 and the fluff pulp 7 from falling offout of the core 2. The sheets 3 are water-pervious at least in a regioncovering the upper surface 8 of the core 2 and water-pervious orwater-impervious in a region covering the lower surface 9 and the sidesurface 11 of the core 2. In the illustrated embodiment, the core 2 isentirely covered with a pair of water-pervious cover sheets 3.

FIG. 2A is a perspective view showing a stock panel 22 and stock sheets23 used for the water-absorbent structure 1 and FIG. 2B is ascale-enlarged diagram illustrating a part of FIG. 2A. In both FIG. 2Aand FIG. 2B, the stock sheets 23 are indicated by imaginary lines. Thestock panel 22 is of a honeycomb structure in which an assembly of thethermoplastic synthetic fibers 4, the polymer particles 6 and the fluffpulp 7 both scattered in the assembly of fibers form partition walls 26and define a plurality of through-holes 27. The stock panel 22 has inaddition to upper and lower surfaces 28, 29 extending in parallel toeach other, side surfaces 31. There are ups and downs repeated in adirection indicated by an arrow X on the upper and lower surfaces 28,29, each of ups and downs extends in a direction indicated by an arrow Yorthogonal to the direction X. The side surfaces 31 include a pair ofside surfaces 31 a on which ups and downs are repeated in a direction Zorthogonal to the direction X as well as to the direction Y and a pairof side surfaces 31 b on which the through-holes 27 are exposed. Thethrough-holes 27 extend parallel to one another in a direction parallelto the upper and lower surfaces 28, 29 between these upper and lowersurfaces 28, 29. Ends of the respective through-holes 27 are exposed inthe side surfaces 31 b. The through-holes 27 are arranged between theupper and lower surfaces 28, 29 so that at least two through-holes 27are aligned in the direction Z. While a cross-sectional shape of thethrough-hole 27 in the stock panel 22 is not specified, a preferredshape thereof is a parallelogram defined by a pair of diagonals 30 a, 30b as seen in FIG. 2B. The diagonal 30 a, one of these two diagonals,extends in the direction X preferably at an angle A of 15 to 45°, morepreferably of 30 to 45° with respect to a pair of opposite sides of theparallelogram and the other diagonal 30 b extends in the direction Z,i.e., between the upper and lower surfaces 28, 29. In other words, thepartition walls 26 are slanted at the angle A of 15 to 45° with respectto the horizontal extending in the direction X. The stock panel 22preferably has a basis weight in a range of 300 to 3,000 g/m², of whichthe thermoplastic synthetic fibers 4 occupy 5 to 80 wt %, thesuper-absorbent polymer particles 6 occupy 10 to 80 wt % and the fluffpulp 7 occupies 5 to 60 wt %.

The thermoplastic synthetic fibers 4 constituting the stock panel 22 areintertwined and/or heat-welded together to form a fibrous assemblyhaving a three-dimensional web formation, the polymer particles 6 andthe fluff pulp 7 are scattered in interstices 32 of the fibers 4. Theseinterstices 32 are significantly smaller than the through-holes 27. Thethermoplastic synthetic fibers 4 have a fineness in a range of 1 to 20dtx and may be selected from the group including short fibers, longfibers, conjugated fibers or crimped conjugated fibers of material whichis water-resistant to retain its elasticity, for example, polyethylene,polypropylene, nylon or polyester.

The through-holes 27 in the stock panel 22 are dimensioned so that thethrough-holes 27 may occupy 10 to 90% of an area on each of the sidesurfaces 31 b and each of the through-holes 27 may have an open areasufficiently large to contain therein at least in order of 10interstices 32 of the fibers 4.

The stock sheet 23 may be formed by a tissue paper, a nonwoven fabric, aperforated plastic film or the like when it should be water-pervious andmay be formed by a water-repellent nonwoven or woven fabric, a plasticfilm or the like when it should be water-impervious.

The stock panel 22 formed in this manner is subjected independently ortogether with the stock sheets 23 to water spray until amoisture-content of the polymer particles 6 reaches 5 to 20 wt %. Afterthe polymer particles 6 has absorbed water to be softened, the stockpanel 22 is compressed in its thickness direction, i.e., the direction Zto a thickness at which the through-holes 27 are flattened and thendried in a compressed state. The compression elastically deforms theassembly of the thermoplastic synthetic fibers 4 having thethree-dimensional web formation so that the component fibers 4 may comeclose to one another. The thermoplastic synthetic fibers 4 lyingadjacent to one another are bonded to one another primarily under anadhesive force of the polymer particles 6 softened in a jelly-like stateand mechanical intertwining with the fluff pulp 7. When the polymerparticles 6 and the fluff pulp 7 are dried, these thermoplasticsynthetic fibers 4, the polymer particles 6 and the fluff pulp 7integrally become rigid and are kept in a compressed state. Before orafter, preferably before compressed, the stock panel 22 is wrapped withthe stock sheets 23 and the stock panel 22 having been wrapped with thestock sheets 23 will be integrated with the stock panel 22 aftercompressed. The stock panel 22 integrated with the stock sheets 23 inthis manner forms the water-absorbent structure 1 of FIG. 1. If thethrough-holes 27 are regularly arranged in the direction X as well as inthe direction Z and the partition walls 26 are formed substantially in auniform thickness as seen in the illustrated embodiment, the stock panel22 will have a substantially uniform thickness after compressed. Sinceeach of the through-holes 27 has a rectangular, more preferablyparallelogrammic cross-section in which one diagonal 30 b of twodiagonals 30 extends in the direction Z, each of the partition walls 26obliquely extends with respect to the direction Z and is substantiallyflattened in the direction Z as the stock panel 22 is compressed in thedirection Z. The partition walls 26 are never buckled even under acompressive force in the direction Z because the partition walls 26 doesnot extend in the direction Z.

FIG. 3 is a perspective view showing the water-absorbent structure 1after the absorption of water. In FIG. 3, the cover sheets 3 are notillustrated and only the core 2 is illustrated. The water-absorbentstructure 1 is adapted to absorb an amount of water permeating throughthe cover sheets 3 thereinto by the polymer particles 6 and the fluffpulp 7. The polymer particles 6 are swollen and softened as theseparticles 6 absorb water and the fluff pulp 7 also is softened as itabsorbs water. Consequently the thermoplastic synthetic fibers 4 are nowfree from immobilization by these polymer particles 6 and the fluff pulp7. Specifically, the fibers 4 having been elastically deformed in thecompressed core 2 are now able to move so as to be restored to the stateprior to compression and swelling of the polymer particles 6 scatteredin the interstices of the fibers 4 enhances enlargement of theinterstices 32 to the initial state prior to compression.Simultaneously, the partition walls 26 having been deformed to flattenthe through-holes 27 can elastically move so as to restore thethrough-holes 27 to the initial shape in the stock panel 22. Arelatively large amount of the polymer particles 6 may be distributedalong respective crossways of the partition walls 26 and in the vicinityof these crossways to promote the partition walls 26 to rise from thecollapsed state and to extend obliquely with respect to the direction Zas these particles 6 absorb water and swell. As the partition walls 26moves to extend obliquely with respect to the direction Z, the core 2swells toward its upper and lower surfaces 28, 29, i.e., in thedirection Z as illustrated in FIG. 3 and the assembly of thethermoplastic synthetic fibers 4 restores its honeycomb constructionillustrated in FIG. 2.

In the water-absorbent structure 1, a variation in the thickness thereofbetween before and after water-absorption depends on its variation dueto swelling of the polymer particles 6 and its variation due torestoration of the honeycomb construction. In the core 2 illustrated inFIG. 3, the partition walls 26 forming the stock panel 22 and thethrough-holes 27 reappear as seen in FIG. 2. In the core 2, the polymerparticles 6 are swollen and a part of them are inside the through-holes27 out of the partition walls 26 in a swollen state. However, thethrough-holes 27 have a sufficient large cross-sectional dimension toprevent the through-holes 27 from being clogged up by the swollenpolymer particles 6. In the water-absorbent structure 1 of FIG. 3 inwhich the honeycomb construction has restored its initial state, aplurality of through-holes 27 extend parallel to one another in thedirection parallel to the upper and lower surfaces 28, 29. With thethrough-holes 27, even if the interstices 32 of the thermoplasticsynthetic fibers 4 in the core 2 are filled with the swollen polymerparticles 6, the water-absorbent structure 1 maintains a highbreathability. Even if the initial water absorption by thewater-absorbent structure 1 causes the polymer particles 6 to form a gelblock, the amount of water permeating thereafter into thewater-absorbent structure 1 can spread via the through-holes 27 downwardas well as laterally into the core 2 and can be absorbed by the polymerparticles 6 even at the corners of the core 2. In this way,substantially all of the polymer particles 6 can be effectivelyutilized. The water-absorbent structure 1 in the state as shown in FIG.1 can be elastically compressed in its thickness direction so that thepartition walls 26 themselves reduce the interstices 32 of the fibers 4and the structure 1 as a whole can be elastically compressed so as toflatten the through-holes 27.

FIG. 2 is a perspective view showing the stock panel 22 and the stocksheets 23 constituting the water-absorbent structure 1 in a mannerdifferent from the manner shown in FIG. 2. The stock sheets 23 areindicated by imaginary lines. In this case, a plurality of honeycombthin leaves 41 having the same composition and construction as the stockpanel 22 of FIG. 2 are used. Each of the thin leaves 41 has thepartition walls 26 and a plurality of through-holes 27 defined by thesepartition walls 26 and preferably has a width W of 5 to 50 mm in thedirection in which the through-holes 27 extend. Each pair of the thinleaves 41 adjacent to each other so that the partition walls 26 of theone thin leaf 41 and the partition walls 41 of the other thin leaf 41are in conformity with each other or so that the partition walls 26 ofthe one thin leaf 41 block a part of the partition walls 26 of the otherthin leaf 41. That is, the through-holes 27 of the one thin leaf 41 areat least partially connected to the through-holes 27 of the other thinleaf 41. Similar to those of FIG. 2, the stock panel 22 and the stocksheets 23 are compressed together to form the water-absorbent structure1.

Use of the thin leaves 41 in the manner as illustrated in FIG. 4 allowsa relatively large water-absorbent structure 1 to be made. Furthermore,when this water-absorbent structure 1 absorbs water and swells, a part42 of the through-holes 27 is exposed on the upper surface 28 asillustrated in FIG. 4 and the amount of water permeating into thestructure 1 after swollen can flow through the part 42 downward into thestructure 1 and easily come in contact with the polymer particles 6lying in the lower region of the structure 1.

FIG. 5 is a partially cutaway perspective view showing a molding die 50used to obtain the stock panel 22 as illustrated in FIGS. 2 and 4. Themolding die 50 comprises a box-like container 51 and a plurality ofsquare pins 52 rising from within the container 51. The thermoplasticsynthetic fibers 4, the super-absorbent polymer particles 6 and thefluff pulp 7 are mixed in a predetermined proportion and fed under theeffect of air stream into the container 51. Then the container 51 isheated or the mixture including the fibers is subjected to hot air blastso that the thermoplastic synthetic fibers 4 are welded one another atcross-points thereof to form the three-dimensional web formation and atthe same time to scatter the polymer particles 6 and the fluff pulp 7 inthis web formation. The assembly of the thermoplastic synthetic fibers 4may be taken out from the molding die 50 to obtain the stock panel 22 ofFIG. 2, which has the through-holes 27 having the shapes correspondingto the pins 52. In the process for making the stock panel 22 in thismanner, the thermoplastic synthetic fibers 4 having a length of 5 to 50mm, more preferably of 5 to 30 mm may be used to obtain the stock panel22 in which the fiber orientation is relatively simple, the fibers 4,the polymer particles 6 and the fluff pulp 7 are rather uniformly mixedwith one another.

Without departing from the scope of the invention, the super-absorbentpolymer particles 6 may be replaced by a fibrous super-absorbentpolymer. It is also possible to use a tissue paper as the stock sheets23 serving to wrap the stock panel 22 and to cover this tissue paperwith a nonwoven fabric or a perforated plastic film from above. Theupper surface 28 and/or the lower surface 29 of the stock panel 22 maybe flat instead of those repeating ups and downs. The water-absorbentstructure 1 according to the invention may be directly used aswater-absorbent product such as a wet wipes or a water-absorbentmaterial or a part thereof used in a disposable wearing article forabsorption and containment of body fluids such as a disposable diaper ora sanitary napkin.

The water-absorbent structure according to this invention is in the formof honeycomb construction in which the fibrous assembly containing thesuper-absorbent polymer scattered therein and normally in a compressedstate. The honeycomb construction is restored as the super-absorbentpolymer is swollen and softened after absorption of water. This featureallows the water-absorbent structure to maintain the desiredbreathability and the compressive elasticity.

1-8. (canceled)
 9. Process for making a water-absorbent structurecomprising a panel-shaped assembly of thermoplastic synthetic fibershaving upper and lower surfaces extending in parallel to each other,cellulose-based fibers and super-absorbent polymer adapted to be swollenafter absorption of water both contained in said assembly of saidthermoplastic synthetic fibers served as water-absorbing materialswherein at least one of said upper and lower surfaces is wrapped withwater-pervious sheets, said assembly comprising a honeycomb constructionadapted to be elastically compressed in a thickness direction and havinga plurality of through-holes extending parallel to one another in adirection parallel to said upper and lower surfaces, each of saidthrough-holes has a cross-sectional dimension larger than any one ofinterstices of said thermoplastic synthetic fibers in said assembly,said process comprises the steps of: compressing said assembly in saidthickness direction while said water absorbing materials are in a wettedcondition so that said through-holes are flattened; drying said waterabsorbing materials so as to maintain said assembly in a compressedstate; and wrapping at least one of said upper and lower surfaces withsaid water-pervious sheets before or after said step of compressing. 10.The process according to claim 9, wherein said super-absorbent polymeris provided in a particulate or fibrous form.
 11. The process accordingto claim 9, wherein said process further comprises the steps of feedinga mixture of said thermoplastic synthetic fibers, said cellulose-basedfibers and said super-absorbent polymer into a molding die and weldingsaid thermoplastic synthetic fibers at crossways thereof within saidmolding die under heating to obtain said assembly.
 12. The processaccording to claim 9, wherein said assembly includes at least two saidthrough-holes aligned in said thickness direction.